Desiree Willis, Technical Writer, The Robbins Company

Factory testing of the EPBM conditioning system

Zhengzhou, a city of 7 million people, is set to become a crossroads for rail commerce in China. The country's future main routes, traveling between Beijing and Guangzhou (North-South), and between Xuzhou and Lanzhou (East-West), will intersect in the city. The plan includes up to four rail lines in Zhengzhou itself, which will eventually link up to the national lines. Excavation of Zhengzhou Metro's new Line 1 tunnels began on September 28, 2010 when the first of two 6.3m (20.7ft) diameter Robbins EPBMs was launched from a 16m (52ft) deep shaft.

The second machine is scheduled for a launch in late October. Both TBMs, for the 11th Bureau of the China Railway Construction Corp. (CRCC), will excavate under downtown Zhengzhou with cover as low as 8m (26ft). The parallel 3.6km (2.2 miles) long tunnels will pass through four intermediate stations between Kaixuan and Tongbo areas of the city. Ground is expected to consist of clay, fine sand, loess, and some pebbles, with little groundwater.

Ready for launch

"The most critical issue for this project," said Steven Zhu, Robbins Project Manager, "is passing under Xi Liu Lake, a large body of water with a depth of 1m to 2m (3.2-6.6ft) and a distance between the bottom of the lake and the top of the tunnel of just 7m (23ft)." The tunnel will also pass beneath building foundations and a highway interchange bridge. In order to reduce settlement, foam and bentonite will be injected for soil conditioning. The advance rate and the material removed will be continuously and closely monitored to prevent subsidence.

As the machines bore, they will line the tunnel with 300mm (12in) thick concrete segments in a 5+1 arrangement. Curves with radii as small as 200m (656ft) will be negotiated using active articulation to prevent segment deformation.

Line 1 of Zhengzhou Metro will consist of 26km (16 miles) of tunnel and 22 stations once complete in 2013. The Zhengzhou Metro company has invested Yuan 10.2 billion (US$1.5 billion) in the six rail lines, which will total 188km (117 miles) by their completion between 2015 and 2030.

These two Robbins EPBMs to Zhengzhou follow several other Robbins machines delivered to China in recent times including the machines to the Jinping-II hydro project; to the Chengdu Metro; and to the Guangfo high-speed intercity railway project between Guangzhou and Foshan.

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